The Impact of Orbital and Clock Errors on Positioning from LEO Constellations and Proposed Orbital Solutions

Two approaches are discussed for the estimation and prediction of the orbits of low earth orbit (LEO) satellites that can be used for navigation. The first approach relays on using a ground monitoring network of stations. The procedures to generate the LEO orbital products in this approach are proposed at two accuracy levels to facilitate different positioning applications. The first type targets producing orbits at meter-level accuracy, defined here as LEO-specific broadcast ephemeris. The second type of products would produce orbits with an accuracy of cm as polynomial corrections to the first type of orbits. Real and simulated LEO satellite data is used for testing, mimicking LEO satellites that can be used for positioning. For the first type of products, it was found that orbital prediction errors play the dominant role in the total error budget, especially in cases of mid and long-term prediction. For the second type of products, the predicted orbits within a short period of up to 60 s generate errors at a few cm, and fitting the corrections with a quadratic polynomial reduced the fitting range errors to the cm level compared to the case of applying a linear polynomial. This level of accuracy can fulfill the requirement for precise point position ing (PPP). The second approach is computing the orbits in real time applying the kinematic or reduced-dynamic mode, where the orbits are computed in the PPP mode using GNSS observations collected onboard LEO satellites and the GNSS orbits and clock products are received through inter-satellite links such as the free-access SouthPAN service in Australia, Galileo HAS, or Beidou (BDS-3, PPP-B2b service). The limitations of this approach and preliminary results are given. Furthermore, the LEO satellite clocks determined together with the orbits in the reduced-dynamic LEO satellite orbit process in near-real-time are also analysed. Finally, the impact of possible orbital and clock errors in the range of decimetres to several meters of LEO satellites on positioning performance is analysed

Satellitenorbit und -ephemeridenbestimmung mit Hilfe von Intersatellitenverbindungen

Global navigation satellite systems like GPS, GLONASS or the future systems like Galileo require precise orbit and clock estimates in order to provide high positioning performance. Within the frame of this Ph. D. thesis, the theory of orbit determination and orbit computation is reviewed and a new approach for precise orbit and ephemeris determination using inter-satellite links is developed. To investigate the achievable accuracy, models of the various perturbing forces acting on a satellite have been elaborated and coded in a complex software package, allowing system level performance analysis as well as detailed evaluation of orbit prediction and orbit estimation algorithms. Several satellite constellations have been simulated, involving nearly all classes of orbit altitude and the results are compared. The purpose of orbit determination in a satellite navigation system is the derivation of ephemeris parameters which can be broadcast to the user community (or the other satellites) and allow easy computation of the satellites position at the desired epoch. The broadcast ephemeris model of both today existing satellite navigation systems, GPS and GLONASS are investigated, as well as two new models developed within this thesis, which are derivates of the GLONASS model. Furthermore, the topic of autonomous onboard processing is addressed. A conceptual design for an onboard orbit estimator is proposed and investigated with respect to the computational load. The algorithms have been implemented. The main benefits of ISL onboard processing, especially with respect to the great potential to ephemeris and clock state monitoring are investigated using complex simulations of failure scenarios. By simulating several types of non-integrity cases, it is showed that one single fault detection mechanism is likely to be insufficient. Within the algorithm design of the onboard processor, a reasonable combination of fault detection mechanisms is presented, covering different fault cases.Globale Navigationssysteme wie GPS, GLONASS oder zukünftige Systeme wie Galileo erfordern die hochpräzise Bestimmung der Orbital- und Uhrenparameter, um hohe Navigationsgenauigkeit bieten zu können. Im Rahmen dieser Dissertation wurde die Theorie der Orbitprädiktion und der Orbitbestimmung erörtert und ein neuer Ansatz für die präzisen Orbitbestimmung mit Hilfe von Intersatelliten-Messungen entwickelt. Um die erreichbare Genauigkeit und Präzision der Orbitbestimmung zu untersuchen, wurden mathematische Modelle der zahlreiche Orbitstörungen erarbeitet und in einem komplexen Software-Paket implemetiert. Dieses bietet die Möglichkeit für Systemstudien von Satellitennavigations-Systemen beliebiger Orbitklassen, sowie zur detaillierten Untersuchung spezieller Fragestellungen der Orbitprädiktion und -bestimmung. Eine Reihe von Simulationen mit existierenden sowie fiktiven Satelliten-Navigations-Systemen wurden durchgeführt, deren Ergebnisse in dieser Arbeit präsentiert werden. Die präzise Orbitbestimmung in einem SatNav-System ist kein Selbstzweck, sondern dient lediglich der Bestimmung der Ephemeridenparameter, die - vom Satellite gesendet - es dem Nutzer-Empfänger erlauben, mit Hilfe einfacher Berechnungen die Position des Satelliten zu ermitteln. Die Ephemeridenformate beider existierender SatNav-Systeme - GPS und GLONASS - wurden untersucht und mit zwei weiteren Formaten verglichen, die im Rahmen dieser Arbeit entwickelt wurden. Desweiteren wurde das Thema der bordautonomen Verarbeitung von Messungen behandelt. Ein konzeptuelles Design für einen Onboard-Prozessor wurde vorgeschlagen und die Algorithmen implementiert. Dabei erfolgte eine Abschätzung der benötigten Prozessorleistung. Einer der Hauptvorteile der bordautonomen Verarbeitung von Intersatellitenmessungen, die Möglichkeit zur Überwachung der Integrität der Ephemeriden und Uhrenparameter, wurde in komplexen Simulationen untersucht. Durch die Simulation verschiedener Fehlerfälle wurde gezeigt, das kein Detektionsmechanismus allein, wohl aber eine sinnvolle Kombination solcher Mechanismen, zur bordautonomen Integritätsüberwachung geeignet sind. Die Ergebissen werden hier präsentiert

THE IMPACT OF ORBITAL AND CLOCK ERRORS ON POSITIONING FROM LEO CONSTELLATIONS AND PROPOSED ORBITAL SOLUTIONS

Two approaches are discussed for the estimation and prediction of the orbits of low earth orbit (LEO) satellites that can be used for navigation. The first approach relays on using a ground monitoring network of stations. The procedures to generate the LEO orbital products in this approach are proposed at two accuracy levels to facilitate different positioning applications. The first type targets producing orbits at meter-level accuracy, defined here as LEO-specific broadcast ephemeris. The second type of products would produce orbits with an accuracy of cm as polynomial corrections to the first type of orbits. Real and simulated LEO satellite data is used for testing, mimicking LEO satellites that can be used for positioning. For the first type of products, it was found that orbital prediction errors play the dominant role in the total error budget, especially in cases of mid and long-term prediction. For the second type of products, the predicted orbits within a short period of up to 60 s generate errors at a few cm, and fitting the corrections with a quadratic polynomial reduced the fitting range errors to the cm level compared to the case of applying a linear polynomial. This level of accuracy can fulfill the requirement for precise point positioning (PPP). The second approach is computing the orbits in real time applying the kinematic or reduced-dynamic mode, where the orbits are computed in the PPP mode using GNSS observations collected onboard LEO satellites and the GNSS orbits and clock products are received through inter-satellite links such as the free-access SouthPAN service in Australia, Galileo HAS, or Beidou (BDS-3, PPP-B2b service). The limitations of this approach and preliminary results are given. Furthermore, the LEO satellite clocks determined together with the orbits in the reduced-dynamic LEO satellite orbit process in near-real-time are also analysed. Finally, the impact of possible orbital and clock errors in the range of decimetres to several meters of LEO satellites on positioning performance is analysed

Precise Orbit Determination of CubeSats

CubeSats are faced with some limitations, mainly due to the limited onboard power and the quality of the onboard sensors. These limitations significantly reduce CubeSats' applicability in space missions requiring high orbital accuracy. This thesis first investigates the limitations in the precise orbit determination of CubeSats and next develops algorithms and remedies to reach high orbital and clock accuracies. The outputs would help in increasing CubeSats' applicability in future space missions

Performances of a GNSS receiver for space-based applications

Space Vehicle (SV) life span depends on its station keeping capability. Station keeping is the ability of the vehicle to maintain position and orientation. Due to external perturbations, the trajectory of the SV derives from the ideal orbit. Actual positioning systems for satellites are mainly based on ground equipment, which means heavy infrastructures. Autonomous positioning and navigation systems using Global Navigation Satellite Systems (GNSS) can then represent a great reduction in platform design and operating costs. Studies have been carried out and the first operational systems, based on GPS receivers, become available. But better availability of service could be obtained considering a receiver able to process GPS and Galileo signals. Indeed Galileo system will be compatible with the current and the modernized GPS system in terms of signals representation and navigation data. The greater availability obtained with such a receiver would allow significant increase of the number of point solutions and performance enhancement. For a mid-term perspective Thales Alenia Space finances a PhD to develop the concept of a reconfigurable receiver able to deal with both the GPS system and the future Galileo system. In this context, the aim of this paper is to assess the performances of a receiver designed for Geosynchronous Earth Orbit (GEO) applications. It is shown that high improvements are obtained with a receiver designed to track both GPS and Galileo satellites. The performance assessments have been used to define the specifications of the future satellite GNSS receiver

Survey of Inter-satellite Communication for Small Satellite Systems: Physical Layer to Network Layer View

Small satellite systems enable whole new class of missions for navigation, communications, remote sensing and scientific research for both civilian and military purposes. As individual spacecraft are limited by the size, mass and power constraints, mass-produced small satellites in large constellations or clusters could be useful in many science missions such as gravity mapping, tracking of forest fires, finding water resources, etc. Constellation of satellites provide improved spatial and temporal resolution of the target. Small satellite constellations contribute innovative applications by replacing a single asset with several very capable spacecraft which opens the door to new applications. With increasing levels of autonomy, there will be a need for remote communication networks to enable communication between spacecraft. These space based networks will need to configure and maintain dynamic routes, manage intermediate nodes, and reconfigure themselves to achieve mission objectives. Hence, inter-satellite communication is a key aspect when satellites fly in formation. In this paper, we present the various researches being conducted in the small satellite community for implementing inter-satellite communications based on the Open System Interconnection (OSI) model. This paper also reviews the various design parameters applicable to the first three layers of the OSI model, i.e., physical, data link and network layer. Based on the survey, we also present a comprehensive list of design parameters useful for achieving inter-satellite communications for multiple small satellite missions. Specific topics include proposed solutions for some of the challenges faced by small satellite systems, enabling operations using a network of small satellites, and some examples of small satellite missions involving formation flying aspects.Comment: 51 pages, 21 Figures, 11 Tables, accepted in IEEE Communications Surveys and Tutorial